In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
The never-ceasing quest for higher data rates in communications networks has led to communications systems employing higher and higher bandwidths. This could make it challenging for terminal devices to achieve a required link budget since the power spectral density (PSD) generally decreases with increased bandwidth for a given total transmit power. This may be particularly challenging for the higher frequency bands considered for the so-called 5th generation telecommunications systems (collectively denoted 5G) since both the path loss and available bandwidth at these frequency bands are high.
For example, it could be challenging to achieve high performance in the uplink (UL), i.e., from terminal devices at the user side of the communications network to network nodes at the network side of the communications network, since the available transmit power in terminal devices may not be sufficient, particularly in handheld terminal devices, such as in so-called user equipment (UE).
One way to improve the UL link budget for terminal devices with high path loss is to reduce the bandwidth in order to obtain sufficiently high PSD at the receiver. A drawback with this approach is that the bandwidth reduction leads to reduced maximum achievable data rate, since this scales linearly with the bandwidth (not considering coding).
Another way to improve the link budget is to use multiple transmit antennas at the terminal device and apply beamforming. A potential issue with this approach is that beamforming requires channel state information (CSI) for the transmit channel. CSI at the transmitter may not always be available. Furthermore, transmit beamforming gain at the terminal device might be difficult to achieve since a rich scattering environment surrounding the terminal device and challenges in antenna design can make the antenna correlation low in the terminal device.
CSI at the transmitter can be obtained by feedback or by reciprocity. If the antenna correlation is low, a feedback approach gives high signalling overhead since no particular structure can be assumed in the quantization of the channel state. On the other hand, reciprocity mandates time-division duplex (TDD) operation and also requires that the receive branches and the transmit branches in the terminal device are calibrated.
An alternative approach to beamforming could be to transmit on a single antenna at the terminal device. However, this will not utilize all the transmit power in the terminal device if there is one power amplifier (PA) per antenna.
Yet another approach could be to use a transmit diversity scheme. However, orthogonal space-time block codes with full rate only exist for two transmit antennas.
Hence, there is still a need for ways to improve the link budget that allows the available transmit effect at the terminal devices to be utilized in an optimal way, without the need of using beamforming.